Although we separate these processes,
as
we will see, both work together to break down rocks and minerals to
smaller
fragments or to minerals more stable near the Earth's surface.

Physical
Weathering

Physical weathering takes place
by a
variety of processes. Among them are:

Development
of Joints - Joints are
regularly spaced fractures or cracks in rocks that show no
offset
across the fracture (fractures that show an offset are called faults).

Joints form as a result of
expansion due
to cooling or relief of pressure as overlying rocks are removed by
erosion.

Joints form free space in
rock by which
other agents of chemical or physical weathering can enter.

Crystal Growth - As water
percolates through
fractures and pore spaces it may contain ions that precipitate to form
crystals. As these crystals grow they may exert an outward
force
the can expand or weaken rocks.

Heat - Although
daily heating and cooling
of rocks do not seem to have an effect, sudden exposure to
high
temperature,
such as in a forest or grass fire may cause expansion and eventual
breakage
of rock. Campfire example.

Plant and Animal Activities -

Plant roots can extend
into fractures and
grow, causing expansion of the fracture. Growth of plants can
break
rock - look at the sidewalks of New Orleans for an
example.

Animals burrowing
or moving through cracks
can break rock.

Frost Wedging - Upon freezing,
there is an increase in the volume of the water (that's why
we
use
antifreeze in auto engines or why the pipes break in New Orleans during
the rare freeze). As the water freezes it expands and exerts
a
force
on its surroundings. Frost wedging is more prevalent at high
altitudes
where there may be many freeze-thaw cycles.

Chemical
Weathering

Since many rocks and minerals
are formed
under conditions present deep within the Earth, when they arrive near
the
surface as a result of uplift and erosion, they encounter conditions
very
different from those under which they originally formed.
Among
the
conditions present near the Earth's surface that are different from
those
deep within the Earth are:

Lower
Temperature (Near the surface T =
0-50oC)

Lower
Pressure (Near the surface P = 1
- several hundred atmospheres)

Higher
free water (there is lots of
liquid
water near the surface, compared with deep in the Earth)

Higher
free oxygen (although O is the
most
abundant element in the crust, most of it is tied up bonded into
silicate
and oxide minerals, at the surface there is much more free oxygen,
particularly
in the atmosphere).

Because of these different conditions,
minerals in rocks react with their new environment to produce new
minerals
that are stable under conditions near the surface. Minerals that are
stable
under P, T, H2O, and O2 conditions near the surface
are, in order of most stable to least stable:

Note the minerals with *. These are
igneous minerals that crystallize from a liquid. Note the
minerals
that occur low on this list, are the minerals that crystallize at high
temperature from magma. The higher the temperature of
crystallization,
the less stable are these minerals at the low temperature found near
the
Earth's surface (see Bowen's reaction series).

The main agent responsible for
chemical
weathering reactions is water and weak acids formed in water

An
acid is solution that has abundant
free
H+ ions.

The
most common weak acid that occurs in
surface waters is carbonic acid.

Carbonic acid is produced in
rainwater
by reaction of the water with carbon dioxide (CO2) gas in the
atmosphere.

Types
of Chemical Weathering Reactions

Hydrolysis - H+
or OH- replaces an ion in the mineral.

Example:

Leaching - ions are
removed by dissolution
into water. In the example above we say that the K+ ion
was leached.

Oxidation - Since
free oxygen (O2)
is more common near the Earth's surface, it may react with minerals to
change the oxidation state of an ion. This is more common in
Fe
(iron)
bearing minerals, since Fe can have several oxidation states, Fe, Fe+2,
Fe+3. Deep in the Earth the most common oxidation state
of Fe is Fe+2.

Dehydration - removal of
H2O
or OH- ion from a mineral.

Complete Dissolution -
all of the
mineral is completely dissolved by the water.

Weathering
of Common Rocks

Rock

Primary Minerals

Residual
Minerals*

Leached Ions

Granite

Feldspars

Clay Minerals

Na+, K+

Micas

Clay Minerals

K+

Quartz

Quartz

---

Fe-Mg Minerals

Clay Minerals + Hematite +
Goethite

Mg+2

Basalt

Feldspars

Clay Minerals

Na+, Ca+2

Fe-Mg Minerals

Clay Minerals

Mg+2

Magnetite

Hematite, Goethite

---

Limestone

Calcite

None

Ca+2, CO3-2

*Residual Minerals = Minerals stable
at the Earth's surface and left in the rock after weathering.

Weathering
Rinds, Exfoliation, and Spheroidal Weathering

When rock weathers, it usually
does
so by working inward from a surface that is exposed to the weathering
process.
This may result in:

Weathering Rinds - a rock
may show
an outer weathered zone and an inner unweathered zone in the initial
stages
of weathering. The outer zone is known as a weathering
rind.
As weathering continues the thickness of the weathering rind increases,
and thus can sometimes be used as an indicator of the amount of time
the
rock has been exposed to the weathering process.

Exfoliation -
Concentrated shells
of weathering may form on the outside of a rock and may become
separated
from the rock. These thin shells of weathered rock are
separated
by stresses that result from changes in volume of the minerals that
occur
as a result of the formation of new minerals.

Spheroidal Weathering - If
joints
and fractures in rock beneath the surface form a 3-dimensional network,
the rock will be broken into cube like pieces separated by the
fractures.
Water can penetrate more easily along these fractures, and each of the
cube-like pieces will begin to weather inward. The rate of weathering
will
be greatest along the corners of each cube, followed by the edges, and
finally the faces of the cubes. As a result the cube will
weather
into a spherical shape, with unweathered rock in the center
and
weathered
rock toward the outside. Such progression of weathering is
referred
to as spheroidal weathering

Different rocks are
composed of different
minerals, and each mineral has a different susceptibility to
weathering.
For example a sandstone consisting only of quartz is already composed
of
a mineral that is very stable on the Earth's surface, and will not
weather
at all in comparison to limestone,
composed entirely of calcite, which will eventually dissolve completely
in a wet climate.

Bedding planes, joints,
and fractures,
all provide pathways for the entry of water. A rock with lots
of
these features will weather more rapidly than a massive rock containing
no bedding planes, joints, or fractures.

If there are large
contrasts in the susceptibility
to weathering within a large body of rock, the more susceptible parts
of
the rock will weather faster than the more resistant portions of the
rock.
This will result in differential weathering.

Slope - On steep slopes
weathering products
may be quickly washed away by rains. On gentle slopes the weathering
products
accumulate. On gentle slopes water may stay in contact with
rock
for longer periods of time, and thus result in higher weathering rates.

Climate- High
amounts of water and higher
temperatures generally cause chemical reactions to run
faster.
Thus
warm humid climates generally have more highly weathered rock, and
rates
of weathering are higher than in cold dry climates.
Example:
limestones in a dry desert climate are very resistant to weathering,
but
limestones in a tropical climate weather very rapidly.

Animals- burrowing
organisms like rodents,
earthworms, & ants, bring material to the surface were it can
be
exposed
to the agents of weathering.

Time - since a rate is how
fast something
occurs in a given amount of time, time is a crucial factor in
weathering.
Depending on the factors above, rates of weathering can vary between
rapid
and extremely slow, thus the time it takes for weathering to occur and
the volume of rock affected in a given time will depend on slope,
climate,
and animals.

Soils
Soils are an important natural resource.
They represent the interface between the lithosphere and the
biosphere
- as soils provide nutrients for plants. Soils
consist of weathered rock plus organic material that
comes from
decaying
plants and animals. The same factors that control weathering
control
soil formation with the exception, that soils also requires the input
of
organic material as some form of Carbon.
When a soil develops on a rock, a soil
profile develops as shown below. These different layers are
not
the
same as beds formed by sedimentation, instead each of the horizons
forms
and grows in place by weathering and the addition of organic material
from
decaying plants and plant roots.

Caliche
- Calcium Carbonate
(Calcite)
that forms in arid soils in the K-horizon by chemical precipitation of
calcite. The Ca and Carbonate ions are dissolved from the upper soil
horizons
and precipitated at the K-horizon. In arid climates the
amount of
water passing through the soil horizons is not enough to completely
dissolve
this caliche, and as result the thickness of the layer may increase
with
time.

Paleosols
- If a soil is buried
rapidly, for example by a volcanic eruption, the soil may be preserved
in the geologic
record as
an
ancient soil called a paleosol.

Soil
Erosion

In most climates
it takes between 80 and 400 years to form about one centimeter of
topsoil
(an organic and nutrient rich soil suitable for agriculture).
Thus
soil that is eroded by poor farming practices is essentially lost and
cannot
be replaced in a reasonable amount of time. This could become
a
critical
factor in controlling world population.